The invention relates to an instrument for implanting expandable cylindrical vascular prostheses, so called stents or endoprostheses. The instrument includes a flexible outer tube and an inner tube arranged therein, with the outer and inner tubes moveable relative to one another.
Blockages of a vessel and constrictions of tubular body lumens, such as for example windpipe, bronchial tubes, esophagus, bile ducts, urinary tracts, blood vessels or the like, can be opened by surgical and non-surgical procedures. Non-surgical procedures involve the insertion of stents or endoprostheses in the area of the stenosis. Stents or endoprostheses are vascular supports which strengthen the inner vessel wall and are introduced through catheter technique into the vessel. Endoprostheses are referred to as ensheathed stents. In the following, the term stent is used consistently.
Stents are available in a wide variety of different plastic, metal and hybrid constructions. Many have fixed end diameters and are self-expanding (DE 91 16 881 U1 or DE 42 40 177 A1).
Other designs involve the option to change the diameter through application of suitable tools, e.g. balloons or expanders to suit the anatomic situation. Such a stent is disclosed in U.S. Pat. No. 5,201,901.
Further known in the art are stents made of so-called shape memory alloy. A shape memory alloy is, for example, nitinol which involves a nickel-titanium alloy, and has two defined states that are temperature-dependent. After pre-treatment, nitinol is martensitic in cold state, i.e. plastically deformable without relevant elastic restoring force. When heated, it changes to an austenitic elastic state. This shape memory characteristic is exploited for self-expansion of the stent.
Various tools and instruments are also known for implanting stents. U.S. Pat. No. 4,580,568 discloses a conventional instrument by which a stent is held together, when inserted in a tube. The stent is freed with the aid of a pusher and positioned. Positioning of the stent in the stenosis is, however, rather difficult because a precise placement of the stent in the stenosis requires a sensitive withdrawal of the instrument as the stent is freed. The stent may hereby shift or even spring away. A recovery of the stent into the instrument during implantation is not possible.
U.S. Pat. No. 5,158,548 proposes the arrangement of the stent at the end of a catheter which is held by a flexible sheath which can be removed by actuating a wire guided in the catheter (see in particular FIGS. 16 to 18). Also this type of instrument does not allow a correction or reversal of the procedure during implantation.
Depending on the design, known instruments for implanting vascular prostheses satisfy their function more or less satisfactorily. There is, however, a desire to enable a positional correction of the stent to a retrieval of the stent into the instrument in vivo during implantation.
Another problem that should be mentioned here relates to the fact that the instruments are normally pre-loaded with a stent. The stents are hereby held compressed within the instrument until implantation. Thus, the material of the self-expanding stent can fatigue with time and loose their spring force so that their function is adversely affected. For that reason, the instruments may not be used after their expiration date.